1. Field of the Invention
This invention relates to a method and apparatus for producing a stencil plate from a heat sensitive stencil sheet having a film by perforating the film with a heating device such as a thermal head, and also relates to a stencil plate obtained thereby. This invention particularly relates to a perforation pattern in which size of perforations is kept adequate without application of large energy or high temperature or decline of heat transfer efficiency in the stencil plate making device. The perforation pattern also decreases perforation configuration irregularity that would locally occur at random or depending on image pattern, and further prevents a molten resin of the film from adhering to heating elements of the thermal head.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The heat sensitive stencil sheet has a thermoplastic resin film (hereinafter also called just xe2x80x9cfilmxe2x80x9d) which has a nature that perforations for penetration of ink can be formed by heating with a heating device such as a thermal head or laser. When the stencil sheet is used for printing, ink passes through the perforations and is transferred onto paper. Various materials are proposed hitherto for the film. For example, JP-A-41-7623 proposes polypropylene, polyamides, polyethylene, and vinyl chloride vinylidene chloride copolymers; JP-A-47-1184 proposes propylene copolymers; JP-A-47-1185 proposes chlorinated polyvinyl chloride; JP-A-47-1186 proposes high crystalline polyvinyl chloride; JP-A-49-6566 proposes propylene a-olefin copolymers; JP-A-49-10860 proposes ethylene vinyl acetate copolymer; JP-A-51-2512 proposes acrylonitrile resins, JP-A-51-2513 proposes polyethylene terephthalate; Japanese Patent No. 1,669,893 proposes polyvinylidene fluoride; and Japanese Patent No. 2,030,681 proposes polyethylene naphthalate copolymers. Among them, films that are presently used for heat sensitive stencil sheets on the market are heat shrinkable films obtained by biaxially stretching a polyethylene terephthalate film or vinylidene chloride copolymer film, mainly for reasons of perforation sensitivity (i.e., performance to give sufficiently large perforations with small quantity of heat) and machine suitability (i.e., unlikelihood to cause wrinkling, loosening, elongation and deformation when the stencil sheet is produced into a stencil plate and used for printing). Especially for stencil printing machines which can automatically produce stencil plates and perform printing, the polyethylene terephthalate film is mainly used.
Alternatively, for forming perforations by means of heat, a film obtained by casting a resin with a low melting point may be used in place of the stretched heat shrinkable film. For example, Japanese Patent No. 1,668,117 and JP-A-62-173296 propose films obtained by casting a synthetic resin solution or emulsion, and JP-A-4-78590 proposes a cast thermoplastic resin film containing a silicone oil. In case of the cast film, it is not thermally shrunken, but since it is made of a resin low in melting point, it can be molten at heated portions to form perforations (hereinafter this film is called xe2x80x9chot-melt filmxe2x80x9d).
However, at present, the hot-melt film is not practically used on the market as a heat sensitive stencil sheet. The main reasons are considered to be low perforation sensitivity, perforation configuration irregularity and low mechanical strength for printing use.
Heat shrinkable films of the heat sensitive stencil sheets currently used on the market for stencil printing machines are about 1.5 to 3 xcexcm in thickness, and encounter no difficulty in stable forming and lamination, in contrary to hot-melt films of 10 xcexcm or less in thickness as disclosed in the Japanese Patent No. 1,668,117 and the like.
In terms of behavior of perforation or migration of molten resins, the hot-melt film relies only on surface tension while the heat shrinkable film relies on heat shrinkage stress which is sufficiently larger than the surface tension. Therefore, the heat shrinkable film has such a higher sensitivity as to allow sufficiently large perforations to be obtained with a smaller heat quantity than the hot-melt film with the same thickness and melt viscosity.
The heat shrinkage stress of the heat shrinkable film clearly depends on a temperature, and thus perforations can be obtained faithfully to a temperature pattern formed on the film, for example, by the heating elements of a thermal head. On the other hand, in case where a hot-melt film is heated and perforated due to surface tension, the temperature pattern of heating elements cannot be accurately reflected by the perforation configuration. The reason is that when resins lowered in viscosity due to melting migrate in accordance with surface tension, it does not always migrate toward low temperature portions far away from the center of each heating element, but can be concentrated near fibers of substrates or can flow irregularly due to a shear caused by its motion relative to the heating element. Therefore, even if a heat sensitive stencil sheet using a hot-melt film is processed into a stencil plate with an opening ratio suitable for printing conditions, uniform perforations are hardly obtained. That is, microscopically, large perforations and small perforations exist together, and it is hard to obtain uniform density, for example, in a solid printed portion of an image.
Furthermore, though the hot-melt films are composed of resins of a low melting point, they must be heated by heating elements to a temperature much higher than the heat shrinkable film, in order to sufficiently induce the migration of the resins with surface tension in very small areas (e.g., pixel density of 300 to 600 dpi) and in a short time (e.g., sub scanning period ranging from 2 to 4 ms) that are ordinarily stencil plate making conditions of stencil plate making devices installed in current stencil printing machines. This causes the heating elements to be deteriorated due to overheat.
Moreover, during printing, the heat sensitive stencil sheet is stressed due to shear with printing paper in the rotating direction of printing drum. A heat sensitive stencil sheet having a cast hot-melt film is generally lower in elastic modulus and rupture strength than a heat sensitive stencil sheet having a stretched heat shrinkable film. Therefore, a heat sensitive stencil sheet having a hot-melt film is more likely to cause deformation of printed images and, as the case may be, more likely to be broken to cause stained images, compared with a heat sensitive stencil sheet having a heat shrinkable film.
For the above reasons, it can be said that heat shrinkable films are and will be mainly used as films for heat sensitive stencil sheets. Therefore, the discussion concerning heat sensitive stencil sheets is hereinafter limited to the heat sensitive stencil sheets using a heat shrinkable film.
The heat sensitive stencil sheet is usually prepared by laminating the above-mentioned film on a porous substrate in order to impart a strength necessary for avoiding elongation, wrinkling (which distorts printed image) and breaking (which stains printed images) due to forces acting when the stencil sheet is mounted to a printing machine and used for printing. The porous substrate provides a heat sensitive stencil sheet with a strength, and allows ink to penetrate through perforations after the stencil sheet has been processed into a stencil plate. It is known that materials for the porous substrate include (1) so-called Japanese paper prepared from natural fibers such as Broussonetia Kazinoki, Edgeworthia chrysantha and Manila hemp, (2) paper-like sheets prepared from regenerated or synthetic fibers of rayon, vinylon, polyester, nylon, etc., (3) mixed paper prepared by mixing the natural fibers of (1) and the regenerated or synthetic fibers of (2), and (4) so-called polyester paper prepared by hot-rolling a thin and soft sheet prepared from a mixture of polyester fibers with non-stretched polyester fibers serving as binder fibers.
A heat sensitive stencil sheet prepared by laminating a film and a porous substrate as mentioned above has a strength sufficient to endure the forces caused by printing action of printing machines, but when ink passes through the heat sensitive stencil sheet, specifically through perforations formed in the film, it can happen that the ink passes unevenly depending on dispersion state of the fibers of the porous substrate, causing printed images to be degraded in uniformity of density. In order to avoid it, a heat sensitive stencil sheet made of a single layer of film is proposed.
Methods for perforating the film of the heat sensitive stencil sheet to obtain a stencil plate include the following methods: (1) the film of the heat sensitive stencil sheet is kept in contact with an original having an image area composed of carbon, and is irradiated with infrared light, so that the film is perforated by the heat generated from the image area; (2) the film of the heat sensitive stencil sheet is kept in contact with a thermal head and is relatively moved whilst the thermal head is caused to generate heat at portions of heating elements corresponding to an original image, so that perforations are made in the film; and (3) a laser beam is modulated in accordance with an original image to scan the film of the heat sensitive stencil sheet, so that perforations are made in the film. Among the above methods, the method using infrared light is limited in kinds of originals, and cannot be used for data editing of documents and images. The method using a laser is not practically applied mainly because of the length of stencil plate making time. Therefore, at present, the method using a thermal head is mainly used.
In the stencil plate making process using a thermal head, numerous perforations two-dimensionally arranged in the main scanning direction and the sub scanning direction are formed. In this case, it is desirable that perforations are made almost equal in shape so that an opening ratio suitable for printing conditions is achieved. If the perforations are uniform in shape, microscopic ink transfer states are uniform in printed image area, particularly in solid printed portions, so that density uniformity is achieved. On the contrary, if the perforations are uneven in shape, microscopic ink transfer states are uneven, and it can happen that thin lines are blurred, that density irregularity occurs in solid printed portions, and that excessively large perforations are formed which cause partially excessive ink transfer, hence set-off. Thus, to obtain perforations uniform in shape by respective heating elements, heating elements with various forms are proposed. Japanese Patent No. 2,732,532 proposes a method of obtaining independent perforations in both the main scanning direction and the sub scanning direction by keeping the pitch in the main scanning direction equal to the pitch in the sub scanning direction, keeping the length of heating elements in the main scanning direction shorter than the length in the sub scanning direction, and keeping the length of the heating elements in the sub scanning direction shorter than the pitch in the sub scanning direction. JP-A-4-314552 proposes a method of preventing that adjacent perforations in the main scanning direction are merged with each other, by disposing cooling members made of material having a large heat conductivity between adjacent heating elements in the main scanning direction. JP-A-6-115042 proposes a method of processing a heat sensitive stencil sheet consisting only of a thermoplastic resin film into a stencil plate using a thermal head in which the length of heating elements in the main scanning direction is kept in a range of 15 to 75% of the pitch in the main scanning direction while the length of the heating elements in the sub scanning direction is kept in a range of 15 to 75% of the pitch in the sub scanning direction.
As for perforation pattern, planar forms (such as diameter, aspect ratio and area) and statistical states (such as average and variation) of perforations only have been discussed, but rim configuration of perforations that gives a desirable ink transfer state can be seen only in the following proposals. Japanese Patent No. 2,638,390 proposes a method of obtaining independent perforations in both the main scanning direction and the sub scanning direction by specifying a relationship between four items; the length of heating elements in the main scanning direction, the length of heating elements in the sub scanning direction, the length of perforations in the main scanning direction and the length of perforations in the sub scanning direction. This patent describes that perforations possess rims. JP-A-6-320700 proposes a perforation method comprising the steps of heating a heat sensitive stencil sheet consisting essentially of a film using a first thermal head from one side thereof and subsequently heating it from the other side thereof using a second thermal head. This patent describes that perforations possess sectional profiles. JP-A-8-20123 proposes a method of making a stencil plate from a heat sensitive stencil sheet consisting essentially of a 3.5 xcexcm or thicker thermoplastic resin film only, in which perforations are formed to be conical in sectional form, with the dimensions of the conical section specified in relation with the pitch in the main scanning direction, in order to eliminate perforation shape irregularity caused by the substrate of the heat sensitive stencil sheet.
The above Japanese Patent No. 2,732,532, JP-A-4-314552, and JP-A-6-115042 may be useful for preventing expansion of perforations caused by merging of adjacent perforations and for making perforations uniform in shape, so that a desirable ink transfer state is realized. However, since perforation behavior of stencil sheets depends on physical properties of films, they cannot be said to be the best methods for controlling the shape of perforations with diverse heat shrinkable films.
Furthermore, though said Japanese Patent No. 2,638,390 and JP-A-6-320700 deal with rims and sectional profiles of perforations, they simply refer to existence of such features of perforations, but do not suggest any influence of the rims and the sectional profiles of perforations on the perforation configuration, or any method for inhibiting decline of heat transfer efficiency or method of achieving perforation configuration uniformity.
Moreover, the stencil plate making method described in said JP-A-8-20123 specifies, as described above, the relation between the dimensions of the conical section and the pitch in the main scanning direction, but it is a method of making a stencil plate from a heat sensitive stencil sheet consisting only of a thick thermoplastic resin film without any porous substrate. However, such a heat sensitive stencil sheet is presently not available as a commercial product, and has various other problems than irregularity of perforation shape. Furthermore, the document does not refer to general heat sensitive stencil sheets including the conventional type consisting of a thermoplastic resin film and a porous substrate in terms of sectional form of perforations formed therein, and does not disclose either a finding that the sectional form and height of rims affect heat transfer efficiency and perforation configuration irregularity.
In the case where it is intended to form through holes with a certain size in a stencil sheet, the resin in each portion to be perforated by a thermal head migrates to the rim portion surrounding each through hole, but it can happen that, depending on, for example, thermal physical properties of the film of the heat sensitive stencil sheet and heating conditions of heating elements of the thermal head, the resin accumulated in the rim portion is often formed as a large bulging from a heated surface of the film.
The bulging portions are kept between the heated surface of the film and the heating elements of the thermal head and act to keep the heated surface of the film and the heating elements farther away from each other. As a result, the efficiency of heat transfer from the heating elements to the film is greatly lowered, making it difficult to form the through holes with a desired size. In the case where the size of the through holes does not reach the desired value, prints become insufficient in density. If it is attempted to achieve the desired size by intensifying the energy applied to the heating elements of the thermal head, the heating elements may be damaged.
On the other hand, the distance between the heated surface of the film and the heating elements necessitated by the formed bulging is different between a solid printed area having numerous perforations and an area adjacent to a non-image area having no perforation. So, the above decline of heat transfer efficiency depends upon an image rate and causes density irregularity in prints. Furthermore, since the bulging portions of rims are kept in pressure contact with the heating elements of the thermal head and transported while being shorn, the planar forms of rims of perforations, i.e., the shape of through holes are distorted, thereby causing microscopic density irregularity and lowering reproducibility of patterns such as characters in prints. In the case where the shape of through holes are remarkably distorted, the through holes of adjacent perforations are merged with each other, and from the thus-formed large through holes, excessive quantities of ink is transferred to paper, causing set-off or the like.
Moreover, it can also happen that the resin of the film deformed by the above shearing comes off to be deposited at a position downstream of the heating elements of the thermal head, and the deposit makes the heating elements and the film kept still farther away from each other, thereby greatly lowering stencil plate making performance.
It is known that these undesirable phenomena are attributable, for example, to the thermal physical properties of the film and the heating conditions of the heating elements of the thermal head, but their relation with the height of the rims bulging around the through holes of perforations has never been discussed. Moreover, no particular finding has been obtained on the factors that determine perforation shapes including the rim of each perforation, necessitating trials and errors.
This invention solves this problem. The object of this invention is to provide a perforation pattern that can keep the size of perforations adequate without requiring large energy application and high temperature in the stencil making device while inhibiting the decline of heat transfer efficiency due to the influence of rims, decreases the perforation configuration irregularity that has locally occurred at random or depending on image pattern, and further prevents the resin of the film from adhering to the heating elements.
The inventors have intensively studied perforation behavior of heat sensitive stencil sheets to achieve the above object, and as a result, found that if perforations are formed to ensure that the height of rims conforms to certain conditions in relation with the pitch between adjacent perforations, perforation configuration irregularity can be inhibited to provide good prints, irrespectively of the thickness and melting point of the film.
According to the first aspect of this invention, there is provided a method for producing a stencil plate, which comprises providing a heat sensitive stencil sheet having a heat shrinkable film, and selectively heating said film with a heating device to form independent dot perforations corresponding to an image in said film, so that each of said perforations has a through hole and a rim surrounding said through hole and bulging on a heated side of said film, and said rim has a height that satisfies the following formulae (1) and (2):
hxe2x89xa64 (xcexcm)xe2x80x83xe2x80x83(1) 
hxe2x89xa60.05{square root over ( )}(pxpy)(xcexcm)xe2x80x83xe2x80x83(2) 
where h denotes said height (xcexcm) in reference to the surface of the film before heated, px denotes a pitch (xcexcm) in a main scanning direction of said heating device, and py denotes a pitch (xcexcm) in a sub scanning direction of said heating device.
According to the second aspect of this invention, there is provided an apparatus for producing a stencil plate from a heat sensitive stencil sheet having a heat shrinkable film, comprising a heating device which selectively heats said film to form independent dot perforations corresponding to an image in said film, so that each of said perforations has a through hole and a rim surrounding said through hole and bulging on a heated side of said film, and said rim has a height that satisfies the following formulae (1) and (2):
hxe2x89xa64 (xcexcm)xe2x80x83xe2x80x83(1) 
hxe2x89xa60.05{square root over ( )}(pxpy)(xcexcm)xe2x80x83xe2x80x83(2) 
where h denotes said height (xcexcm) in reference to the surface of the film before heated, px denotes a pitch (xcexcm) in a main scanning direction of said heating device, and py denotes a pitch (xcexcm) in a sub scanning direction of said heating device.
According to the third aspect of this invention, there is provided a stencil plate which comprises a heat shrinkable film having independent dot perforations corresponding to an image, said perforations being formed by selectively heating said film with a heating device, wherein each of said perforations has a through hole and a rim surrounding said through hole and bulging on a heated side of said film, and said rim has a height that satisfies the following formulae (1) and (2):
hxe2x89xa64 (xcexcm)xe2x80x83xe2x80x83(1) 
hxe2x89xa60.05{square root over ( )}(pxpy)(xcexcm)xe2x80x83xe2x80x83(2) 
where h denotes said height (xcexcm) in reference to the surface of the film before heated, px denotes a pitch (xcexcm) in a main scanning direction, and py denotes a pitch (xcexcm) in a sub scanning direction.
According to the fourth aspect of this invention, there is provided a stencil sheet which comprises a heat shrinkable film destined to have independent dot perforations corresponding to an image by selectively heating said film with a heating device, so that each of said perforations has a through hole and a rim surrounding said through hole and bulging on a heated side of said film, and said rim has a height that satisfies the following formulae (1) and (2):
hxe2x89xa64 (xcexcm)xe2x80x83xe2x80x83(1) 
hxe2x89xa60.05{square root over ( )}(pxpy)(xcexcm)xe2x80x83xe2x80x83(2) 
where h denotes said height (xcexcm) in reference to the surface of the film before heated, px denotes a pitch (xcexcm) in a main scanning direction, and py denotes a pitch (xcexcm) in a sub scanning direction.